The present invention relates to a color cathode ray tube and particularly to a shadow mask type color cathode ray tube having an improved resolution capability. Color cathode ray tube such as color picture tubes and display tubes have been widely used as receivers of TV broadcasting or monitors in information processing equipment because of their high-resolution capability.
Generally, such color cathode ray tubes comprise a phosphor screen formed on an inner surface of a faceplate of a panel portion of an evacuated envelope, a shadow mask having a multiplicity of electron beam apertures and spaced from the phosphor screen within the panel portion, an electron gun of the in-line type for projecting electron beams toward the phosphor screen and housed in a neck portion of the evacuated envelope, and a deflection yoke mounted around a funnel portion of the evacuated envelope.
FIG. 6 is a schematic cross sectional view for explaining a construction of a shadow mask type color cathode ray tube as an example of a color cathode ray tube to which the present invention is applicable. In FIG. 6, reference numeral 20 is a faceplate, 21 is a neck, 22 is a funnel for connecting the faceplate 20 and the neck 21, 23 is a phosphor screen serving as an image display screen formed on an inner surface of the faceplate 20, 24 is a shadow mask serving as a color selection electrode, 25 is a mask frame for supporting the shadow mask 24 and for forming a shadow mask assembly, 26 is an inner shield for shielding extraneous ambient magnetic fields, 27 is a suspension spring mechanism for suspending the shadow mask assembly on studs embedded in the inner sidewall of the faceplate 20, 28 is an electron gun housed in the neck 21 for projecting three electron beams Bs (xc3x972) and Bc, 29 is a deflection device for deflecting the electron beams horizontally and vertically, 30 is a magnetic device for adjusting color purity and centering the electron beams, 31 is a getter, 32 is an internal conductive coating, and 33 is an implosion protection band.
The evacuated envelope is formed of a faceplate 20, a neck 21 and a funnel 22. The magnetic deflection fields generated by the deflection device 29 deflect the three in-line electron beams emitted from the electron gun 28 horizontally and vertically to scan the phosphor screen 23 in two dimensions. The three electron beams Bc, Bsxc3x972 are modulated by the green signal (center beam Bc), the blue signal (side beam Bs) and the blue signal (side beam Bs), respectively, and after being subjected to color selection by beam apertures in the shadow mask 24 disposed immediately in front of the phosphor screen 23, impinge on respective phosphor elements of red, green and blue colors of the tricolor mosaic phosphor screen 23 to reproduce the intended color image.
FIGS. 7A to 7C are illustrations of a construction example of the in-line type electron gun applicable to the color cathode ray tube shown in FIG. 6, FIG. 7A is a horizontal sectional view thereof, and FIG. 7B is a schematic sectional view of the major portion of FIG. 7A, taken along the VIIBxe2x80x94VIIB, and FIG. 7C is a schematic sectional view of the major portion of FIG. 7A, taken along the VIICxe2x80x94VIIC. In FIG. 7A, reference numerals 1a to 1c are cathode structures, 2 is a control grid electrode, 3 is an accelerating electrode, 4 is a focus electrode, 5 is an anode, 6 is a shield cup, 41 is a first focus sub-electrode, 42 is a second focus sub-electrode, and the first and second sub-electrodes 41, 42 form a focus electrode 4. Vertical plates 411 are attached to the first focus sub-electrode 41 on the second focus sub-electrode 42 side thereof such that they sandwich each of three electron beams horizontally and they extend toward the second focus sub-electrode 42, a pair of horizontal plates 421 are attached to the second focus sub-electrode 42 on the first focus sub-electrode 41 side thereof such that they sandwich three electron beams vertically and they extend toward the first focus sub-electrode 41, and the vertical plates 411 and the horizontal plates 421 form a so-called electrostatic quadrupole lens. The correction plate electrode 422 with a beam aperture for each of the three electron beams is disposed within the second focus sub-electrode 42 and the correction plate electrode 51 with a beam aperture for each of the three electron beams is disposed within the anode 5.
The vertical plates 411 and the horizontal plates 421 of the electrostatic quadrupole lens, as respectively shown in FIGS. 7B and 7C, are such that the vertical plates 411 are comprised of four plates 411a, 411b, 411c and 411d arranged in such a manner as to sandwich side beam apertures 41s and a center beam aperture 41c in the first focus sub-electrode 41 individually and horizontally and the horizontal plates 421 are comprised of a pair of plates 421a and 421b arranged in such a manner as to sandwich side beam apertures 42s and a center beam aperture 42c in the second focus sub-electrode 42 in common and vertically.
The cathode structures 1a to 1c, the control grid electrode 2 and the accelerating electrode 3 form an electron beam generating section. Thermoelectrons emitted from the heated cathode structure 1 are accelerated toward the control grid electrode 2 by an electric potential of the accelerating grid electrode 3 and form three electron beams. The three electron beam pass through the apertures in the control grid electrode 2, and the apertures in the accelerating electrode 3, and after having astigmatism corrected by the electrostatic quadrupole lens disposed between the first and second focus sub-electrodes 41 and 42, and enter the main lens formed between the second focus sub-electrode 42 and the anode 5. The three electron beams are focused by the main lens, and after being subjected to color selection by the shadow mask, and impinge upon the intended respective phosphor elements of the phosphor screen and produce the bright spots of the intended colors.
The first focus sub-electrode 41 is supplied with a fixed voltage Vf1 and the second focus sub-electrode 42 is supplied with a dynamic voltage Vf2+dVf which is a fixed voltage Vf2 superposed with a voltage dVf varying in synchronism with deflection angles of the electron beams. The anode 5 is supplied with the highest voltage Eb via the internal conductive coating 32 (see FIG. 6) coated on the inner surface of the funnel 22.
With this construction, the curvature of the image field is corrected by varying the lens strength with the deflection angle of the electron beams and astigmatism is corrected by the electrostatic quadrupole lens such that the focus length of the electron beams and the shape of the beam spots are controlled to provide good focus over the entire phosphor screen.
To obtain a normal round beam spot at the center of the phosphor screen, the horizontal and vertical effective lens diameters are approximately equalized with each other for each of the three electron beams by optimization in terms of the dimensions of the single openings common for the three electron beams in the second focus sub-electrode 42 and the anode 5 for forming the main lens portion, the dimensions of the beam apertures in the correction plate electrodes 422, 51 disposed within the second focus sub-electrode 42 and the anode 5, and the axial distances between the correction plate electrodes 422, 51 and the single openings in the second focus sub-electrode 42 and the anode 5 incorporating the correction plate electrodes 422, 51.
With such a lens, the resolution capability of the electron beams scanning the phosphor screen was improved and reproduced the high quality image.
The prior art as described above is disclosed in Japanese Patent Application Laid-open Publication No. Hei 2-189842, for example.
Focus characteristics of cathode ray tubes are greatly influenced by the width of horizontal scan lines. In the prior art electron guns, the horizontal and vertical effective lens diameters of the main lens are equalized with each other and the problem arises in that the maximum lens diameter of the main lens is limited by the smaller one of the maximum allowable horizontal and vertical lens diameters of the main lens which are limited by the horizontal or vertical dimension of the structure of the electron gun housed in the neck portion of the cathode ray tube.
Generally, the lens dimension is limited more rigidly in the horizontal direction in which the three in-line electron beams are arranged, and the vertical lens dimension is made so smaller as to be equal to the horizontal lens dimensional though the vertical lens dimension can be increased. Therefore the vertical diameter of an electron beam spot on the phosphor screen cannot be decreased compared with its horizontal diameter and this causes a problem in that it is difficult to reduce the width of the horizontal scan lines.
Also there is a problem in that, if eccentricity of the electrodes is caused in the manufacturing process such as the assembling of the electron gun and the electron beams do not pass through the center of the main lens, the vertical diameter of the beam spot at the phosphor screen increases as much due to vertical eccentricity as its horizontal diameter increases due to horizontal eccentricity, although the increase in the vertical diameter of the beam spot due to the vertical eccentricity can be suppressed to a smaller value.
An object of the present invention is to solve the above-mentioned problems of the prior art and to provide a color cathode ray tube capable of a high resolution image display by reducing the vertical diameter of the electron beam spots on the phosphor screen.
To accomplish the above object, in accordance with an embodiment of the present invention, there is provided a color cathode ray tube comprising an evacuated envelope comprising a panel portion, a neck portion and a funnel portion for connecting the panel portion and the neck portion, a three-color phosphor screen formed on an inner surface of the panel portion, a shadow mask having a multiplicity of apertures therein and spaced from the phosphor screen, a three-beam in-line type electron gun. housed in the neck portion, the three-beam in-line type electron gun including an electron beam generating section for generating three electron beams and a main lens section for focusing the three electron beams on the three-color phosphor screen, and a deflecting device mounted in a vicinity of a transition region between the funnel portion and the neck portion for scanning the three electron beams on the three-color phosphor screen, the main lens section comprising a focus electrode and an anode facing the focus electrode, each of the focus electrode and the anode comprising an electrode having a single opening common for the three electron beams in an end thereof facing each other and a plate electrode disposed therein, set back from an end thereof facing another of the focus electrode and the anode and for forming three beam apertures for passing the three electron beams respectively, the focus electrode comprising at least one first sub-electrode adapted to be supplied with a first focus voltage and at least one second sub-electrode adapted to be supplied with a second focus voltage, one of the at least one first sub-electrode and the at least one second sub-electrode facing the anode, the second focus voltage being a fixed voltage superposed with a dynamic voltage varying with deflection of the three electron beams, an electrostatic quadrupole lens being formed between facing ends of a first one of the at least one first sub-electrode and a first one of the at least one second sub-electrode facing the first one of the at least one first sub-electrode, and a following inequality being satisfied: V1 greater than Hxe2x88x922xc3x97S where V1 is a vertical diameter of the single opening, H is a horizontal diameter of the single opening, S is Pxc3x97L/Q, P is a horizontal center-to-center spacing between adjacent phosphor elements at a center of the three-color phosphor screen, Q is an axial spacing between the three-color phosphor screen and the shadow mask at the center of the three-color phosphor screen, and L is an axial distance between the shadow mask and the single opening in the focus electrode.